1. Field of the Invention
The present invention relates to a load-following hybrid power generating system comprising a fuel cell and a rechargeable storage battery coupled together via a DC-DC converter in which a reactant raw material is processed before the reactant is supplied to the fuel cell.
2. Description of the Related Art
In applications where electricity demand may vary rapidly over time, fuel cell based power generating systems are typically configured as hybrid systems, comprising both a fuel cell power supply and a load levelling power supply such as a rechargeable storage battery. Such a hybrid system is desired if the response time of the fuel cell side of the system is not fast enough to accommodate sudden increases in power demand. In that case, sudden increases in power demand are met by power delivered from the storage battery. With time, the fuel cell side of the system responds to meet the demand and the storage battery is then recharged appropriately using power from the fuel cell. An example of a system where electricity demand may vary rapidly over time is an uninterruptible power supply (UPS) that is designed to back up another power supply without interruption during power outages.
Depending on the power requirements of the applied external load, the generating system power outputs may simply be connected across the storage battery, or instead the system may contain a power conditioning system between the storage battery and the system power outputs. For instance, an inverter may be used to convert DC power from the storage battery into AC. In the generating system, the fuel cell is also electrically connected to the storage battery. However, since the voltage characteristics of the fuel cell and storage battery differ during operation, the typical hybrid system employs a DC-DC converter to couple the fuel cell and the storage battery together (the fuel cell and the battery being electrically connected across the inputs and outputs of the DC-DC converter respectively). In this way, DC current produced by the fuel cell at the fuel cell voltage may be converted to DC current at an appropriate voltage for the storage battery and for the applied external load. The generating system employs some means for determining the power required to supply the external load and to appropriately recharge the storage battery, and uses this information to control the power produced by the fuel cell. For instance, a desired current output from the DC-DC converter may be determined and this information used to control the reactant supplies to the fuel cell and to control the DC-DC converter such that the DC-DC converter applies an appropriate load to the fuel cell.
In providing reactants for the fuel cell, the generating system may also include certain reactant processors that convert raw materials into reactants suitable for the fuel cell. For instance, hybrid systems may include a reformer system that converts a supply of a hydrocarbon fuel (e.g., methane or methanol) into hydrogen reactant for the fuel cell. The amount of material processed by these processors would thus also be controlled in accordance with the power required by the external load and the storage battery. However, for processors like reformer systems, there can be a significant time lag between signaling for a change in the rate of processed reactant and actually obtaining the desired rate of processed reactant. Thus, for a period, the DC-DC converter may draw more or less current from the fuel cell than would be desired on the basis of the actual reactant supplies available to the fuel cell. That is, when the current drawn by the DC-DC converter suddenly increases, the fuel cell may operate at an undesirably high overvoltage for some time until more processed reactant is available. Producing power under such starved operating conditions is inefficient and, in the extreme, may possibly result in damage to the fuel cell. Conversely, when the current drawn by the DC-DC converter suddenly decreases, more reactant is supplied to the fuel cell than is needed for the current drawn. Thus, reactant may go unconsumed, which is also inefficient.
Unpredictable variations in power demand thus pose challenges in controlling such systems such that the demand for electricity is met while still operating efficiently.
The present invention relates to an improved fuel cell/storage battery hybrid power generating system wherein a reactant processor is controlled on the basis of the determined current requirements of the external load and of the storage battery, and wherein the current drawn from the fuel cell by the DC-DC converter is controlled on the basis of the amount of processed reactant that is actually available from the processor.
The improved power generating system includes a fuel cell, a reactant processor for processing a reactant raw material into one or more reactant streams for the fuel cell, a DC-DC current converter, a rechargeable storage battery, a current determiner for determining a desired output current from the DC-DC current converter, and system power outputs for outputting power to an external load. The reactant processor has an inlet for receiving the reactant raw material and an outlet that is fluidly connected to the reactant inlet of the fuel cell. The fuel cell is electrically connected across the current inputs of the DC-DC current converter while the battery is electrically connected across the current outputs of the DC-DC current converter. An output signal from the current determiner is provided as a setpoint input to the reactant processor.
The improved system additionally comprises a rate determiner that is used in the control of the DC-DC converter. The rate determiner determines the rate that reactant is supplied from the reactant processor outlet and provides an output signal to a setpoint input of the DC-DC current converter. The current drawn from the fuel cell is controlled by adjusting the input impedance of the DC-DC current converter in accordance with the signal at the DC-DC current converter setpoint input. In particular, the input impedance may be adjusted such that the reactant is consumed in the fuel cell at a rate proportional to the rate that reactant is supplied from the processor. For purposes of controlling the DC-DC current converter, the system can also comprise a suitable means for measuring the actual current drawn from the fuel cell (e.g., an ammeter measuring the input current to the DC-DC current converter and providing a signal thereto). Circuitry within the converter can then compare the requested current drawn (represented by the setpoint input) to that actually drawn and adjust the input impedance of the converter appropriately.
The rate determiner can directly measure the rate that processed reactant is provided or can indirectly calculate it instead, based on other measured operation variables (e.g., the rate of hydrogen produced by a reformer may be deduced from the amount of methane supplied to the reformer and from the amount of unreformed methane present in the product reformate, as measured by a suitable methane concentration sensor, such as an infrared detector). The rate determiner thus can comprise, for instance, a reactant rate sensor (e.g., a flowmeter) in the fluid connection between the reactant processor outlet and the reactant inlet of the fuel cell. Alternatively, the rate determiner can comprise a computing unit for calculating the reactant rate supplied from the reactant processor.
The rate at which the reactant processor processes the reactant raw material is adjusted in accordance with the reactant processor setpoint input (i.e., the output signal from the current determiner). The current determiner in the system may comprise a load ammeter measuring current directed to the external load, a battery ammeter measuring current through the storage battery, a charge controller receiving an input signal from the battery ammeter, and a computing unit receiving input signals from the charge controller and the load ammeter and providing an output signal to a setpoint input of the reactant processor. The computing unit may sum input signals from the charge controller and the load ammeter and then output the sum as the output signal provided to the setpoint input of the reactant processor.
The system can also comprise a suitable means for measuring the output current from the DC-DC current converter (e.g., an ammeter) that then can be used to provide a feedback control signal to the reactant processor. Additionally, a signal from an ammeter measuring the current drawn from the fuel cell (i.e., the input current to the DC-DC current converter) may be desirably used as a control signal to the reactant processor. For instance, in the event that the fuel cell is unable to generate the requested current and the current drawn falls below a threshold value, it may be desirable to slow the reactant processing rate until the situation is corrected.
Under certain conditions, it may be desirable to deliberately interrupt the current drawn from the fuel cell (e.g., when a cell reversal or ground fault condition is detected). To accomplish this, the system may include a switch for interrupting current flow to the current inputs of the DC-DC current converter, and a fuel cell monitor connected to the fuel cell and controlling the switch.
In the improved power generating system, a preferred DC-DC current converter is a Cuk-type converter. Such converters advantageously can electrically isolate the fuel cell from the storage battery. Further, such converters are useful for obtaining reduced ripple in the output current.
An application for the improved power generating system is to serve as an uninterruptible power supply. AC power may be provided from the system by additionally incorporating an inverter therein with the current inputs and outputs of the inverter electrically connected across the storage battery and the system power outputs respectively.
The invention is also of benefit in systems in which the raw material for either the fuel or the oxidant is processed into a reactant (e.g., hydrogen or oxygen) for the fuel cell. In the case of the fuel, examples of reactant processors include a reformer system, a pressure swing adsorption system, or a pressure reducer (e.g., pressure regulating valve). In the case of the oxidant, examples of reactant processors include a compressor (e.g., turbo compressor, roots blower, water ring compressor) or a pressure swing adsorption system.